Proceedings of the Institution of Mechanical Engineers,Part L: Journal of Materials: Design and Applications

A numerical modelling strategy for the direct pin pressing process of metallic pins into continuous fibre-reinforced thermoplastic organosheets is developed. The joining process is performed above the thermoplast’s melting temperature, altering the initial material structure of the composite by fibre rearrangement, which in turn influences the load-bearing capacity of the joint. Therefore, the modelling strategy aims at predicting the resultant material structure after pin pressing. The modelling approach considers both the textile architecture and the process parameters (temperature, tool velocity). A sub-meso modelling framework for the fibres based on a multi-filament approach is used. The interaction between fibres and the thermoplastic melt, as well as the matrix flow, is modelled using the Arbitrary Lagrangian Eulerian method. This allows for the prediction of matrix-rich zones and fibre rearrangement around the pin. The promising results show a good agreement of the resultant material structure in terms of compaction and fibre volume content around the pressed pin. Characteristic parameters show an underestimation of the laminate thickness below the pin. Moreover, an evaluation method for evaluating the orientation changes of the virtual multi-filaments is developed and presented to observe and assess fibre rearrangement and fibre volume content in detail during the numerical process simulation. It can be seen that only fibres around the pin are displaced and not in the whole molten area. Furthermore, it can be observed in detail that the initial position of the fibres in relation to the pin determines whether the fibres are displaced in the in-plane or out-of-plane direction.

Lightweight design by using low-density and load-adapted materials can reduce the weight of vehicles and the emissions generated during operation. However, the usage of different materials requires innovative joining technologies with increased versatility. In this investigation, the focus is on describing and characterising the failure behaviour of connections manufactured by an innovative thermomechanical joining process with adaptable auxiliary joining elements in single-lap tensile-shear tests. In order to analyse the failure development in detail, the specimens are investigated using in-situ computed tomography (in-situ CT). Here, the tensile-shear test is interrupted at points of interest and CT scans are conducted under load. In addition, the interrupted in-situ testing procedure is validated by comparing the loading behaviour with conventional continuous tensile-shear tests. The results of the in-situ investigations of joints with varying material combinations clearly describe the cause of failure, allowing conclusions towards an improved joint design.

Adhesive bonding is a commonly used technology for joining dissimilar materials. However, production-related effects on the performance of the bondline have to be considered for an accurate joint design. In case of an adhesive joint in an automotive multi-material body in white, these effects arise from the so-called mismatch in thermal expansion coefficients, which leads to distortions of the adhesive in an uncured state or even damage in the cured one. The distortion of the uncured adhesive in the normal direction is called the ‘viscous fingering effect’, which reduces the adhesively bonded cross section by changing the adhesive bondline's shape to thin ‘fingers’ and therefore influences the materials properties. To investigate the effect of viscous fingering on the modulus, strength and energy release rate, linear butt bonded specimens and Tapered Double Cantilever Beams (TDCB) with different elongations of the adhesive bondline in the viscous state are investigated. The results are used to parameterize a cohesive zone model (CZM) and perform numerical analysis of the TDCB specimen for validation and to build up a model of a dissimilar joint consisting of a steel hatprofile and an adhesively bonded aluminum panel subjected to thermal distortions.

Metal foils are being widely used, from the chemical or electronics sector to household appliances. The joining of these foils by adhesive bonding is often the preferred method due to discolouring and warping under the thermal stresses of other joining methods, such as welding. However, long curing times are a disadvantage of adhesive bonding compared to welding. The use of electromagnetic induction is a promising solution for accelerated curing. This work investigates induction heating for accelerated curing of 1-C epoxy adhesives for bonding of thin nickel foils. Process parameters for rapid curing of the adhesives were determined based on reaction kinetics using differential scanning calorimetry measurements. According to those results peel test specimens were fabricated, and the peel resistance was evaluated using a 90° peel load.

The Cu–6Al–2Ni alloy has much higher ultimate tensile strength compared to pure copper and may potentially replace it in the dissimilar joints between titanium alloys and stainless steels. Laser welding of aluminum bronze to stainless steel has not been reported in the scientific literature, which motivated the present weldability study of Cu–6Al–2Ni/316L dissimilar joint with a continuous ytterbium Yb:YAG laser. Different laser spot offsets from the joint line were selected in order to produce the joints with various dilutions of welded materials. Scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS) probe and X-ray diffraction (XRD) analyses of the melted zones were performed, along with microhardness measurements and tensile testing. The phase evolution in the obtained microstructures was evaluated using Thermo-Calc software. For the dilutions ranging from 23 to 63 at.% Cu, the melted zones showed globular microstructures with primary and secondary phase separation due to the miscibility gap existing in the Cu–Fe system. Lower Cu contents resulted in cellular γ-Fe structures with rare globular Cu-rich inclusions. The XRD analysis indicated the presence of ∼10% of ternary AlFe₂Ni phase, however, it did not harm the mechanical properties of the welds. According to Thermo-Calc, this phase is formed from γ-Fe during the cooling process. Microhardness measurements did not indicate the embrittlement of the melted zones, which can be explained by the submicronic dispersion of AlFe₂Ni. The welds exhibited a ductile fracture in Cu–6Al–2Ni at ultimate tensile strength of 350–420 MPa in a wide range of laser offsets, which is much higher than previously reported results for pure copper/316L joints.

Ultrasonic metal welding is a well-established solid state joining process for electrical applications. The process relies on the friction between workpieces and welding tools for joint formation. This friction is generated by the process force and the ultrasonic oscillation of the welding tools imposed on the workpieces. At such high frequencies, the occurrence of resonances in actual workpiece geometries is not surprising. It is known that critical dimensions in length and width lead to nearly no bond, depending on the welding frequency and the mechanical properties of the material. In real applications, this limits the possible designs of terminals and leads to extensive testing of clamping devices. It is also known that machine learning (ML) models for quality prediction based on power signals or tool oscillation can account for changes in welding position. In this study, we investigated the impact of part resonance and antiresonance on horn and anvil oscillation, power consumption and bond strength to identify typical behaviors induced by the workpieces. The influence of material thickness and roughness was considered, and numerical analysis of the natural frequencies of the workpieces was conducted. It can be shown that the results allow a distinction between the welding positions and workpiece geometries without directly measuring the oscillation patterns of the workpieces, allowing a simple validation of geometry weldability and clamping device in applications. Furthermore, the investigation allows the knowledge based specific deduction of signal parameters for future ML models, allowing a consideration of welding position and workpieces geometry with reduced test data.

Resistance spot welding (RSW) is an economic, robust welding process, which is easy to automate and widely used in automotive industry. In this work, the resistance spot weldability of die-cast aluminum alloys EN AC-AlSi7MnMg, EN AC-AlSi9Mn and EN AC-AlSi10MnMg-T6/T7 to wrought aluminum alloy EN AW-AlSi1MnMg-T6 is investigated when applying the standard welding profile as recommended in VDA 238-401 considering main challenges in welding aluminum die castings, for example, porosity and inhomogenities. Weldability is best for EN AC-AlSi10MnMg-T6/T7, but the results show limited applicability of VDA 238-401 in general, especially for high total sheet thicknesses, because of invalid electrode force favouring spatter formation and weld spot irregularites. Still, all spot welds between die castings and 2 mm wrought aluminum sheets reach the recommended shear tension forces for spot diameter d_w of 5·√t (2 mm: 5.95 kN; 3 mm: 8.96 kN; according to DIN EN ISO 18595), although being afflicted with solidification porosity and liquation cracking. Gas porosity is not observed and solidification porosity is only found to be significant for weld spot failure when being located close to the joining level, which is observed at welding depths of approximately 50% into the wrought aluminum sheet. Here, a failure path across the nugget is found. Weld porosity apart from the joining level is less significant and failure path is found along the fusion line between nugget and wrought aluminum. Instead, hardness difference between nugget and HAZ is the main reason for weld spot failure. Although the applied welding current profile is not suited best for RSW of mixed joints between aluminium die castings and wrought aluminium alloys, the results confirm its potential, for example, a significantly reduced gas porosity compared to applied fusion welding processes.

One possible option for increasing the fatigue strength of welded joints is the use of so-called low transformation temperature (LTT) alloys. The aim is to introduce residual compressive stresses into the weld to counteract crack initiation and propagation. Until now, there has been no application of an LTT effect to high-performance welding processes such as the laser beam submerged arc hybrid welding process (LUPuS hybrid). First, the LUPuS hybrid single-wire process was further developed into the LUPuS tandem hybrid process. This makes it possible to equip the two submerged arc welding torches with different commercially available filler wires. The aim of the work is to further develop the LUPuS tandem hybrid welding process to enable the use of the LTT effect. The in situ alloying process for obtaining the LTT effect from commercially available material combinations was extended to the two-wire process. The alloy obtained was investigated by means of energy dispersive x-ray spectroscopy and hardness measurements and the influence on residual stresses was determined by the borehole method supported by electronic speckle pattern interferometry.

Localised heat input, as it occurs in welding with moving heat sources, induces residual stresses and distortion in materials. The quantitative determination of residual stress evolution is difficult. Despite existing models, residual stress build-up with temperature progression is not fully understood. High-flux density X-rays from a synchrotron source allow the measurement of local strains in materials and improve the resolution of stress gradients as it permits small measurement volumes (Gibmeier et al., 2014). A laser beam welding process was used to perform linear bead-on-plate welds on bar steel samples. The X-ray diffraction system recorded the transient strain evolution. Multiple repetitions at different locations in the specimen were combined to develop a map of the strains present within the specimen. The temperature was measured locally at the surface of the sample. As the strain was determined within a measurement volume inside the sample, the temperature history over time had to be obtained as well. A numerical model was employed to determine the temperature inside the measurement volume. This model was calibrated using the transient surface temperatures and metallographic cross-sections. The result was a representation of the local strain superimposed on the temperature distribution. Analysis of this data correlation showed that a strain maximum occurs as a function of time and distance from the heat source, which is likely to coincide with the austenite-ferrite phase transformation temperature.

In the area of plant engineering, steel components are provided with a wear protection coating for efficient use to protect them against corrosive, tribological, thermal and mechanical stresses. The use of innovative ultrasound-assisted milling processes and plasma-welded nickel- and cobalt-based wear protection coatings are being investigated to determine how more favourable machinability can be achieved while retaining the same wear protection potential. The focus is on the NiCrSiFeB alloy, which is intended to replace CoCr alloys in the area of screw machines. The utilization of ultrasonic-assisted milling for the machining of coating materials is a novel approach. The modification of hard facing layers in terms of microstructure and precipitation morphology as well as suitability for machining is investigated and compared with the CoCr alloy. The alloy modifications are generated by a PTA process by systematically adjusting the preheating and interpass temperatures, a crack-free wear-resistant layer can be generated, which is subsequently machined by a milling process. In addition to the crack-free properties, the microstructure, the bonding as well as the mixing between the NiCrSiFeB alloy and a 1.8550 as well as between the CoCr alloy and a 1.4828 are analysed and compared in the joining areas. In addition, heating and cooling rates are determined and a chemical analysis of the weld metals is performed. Furthermore, it was found that the build-up layers of NiCrSiFeB alloy are more difficult to machine using the milling process than the CoCr alloy, as higher milling forces are required.

In-situ analysis of dissimilar laser welding in overlap configuration, which finds the most frequent application in industry, attracts an increasing attention of the research community. In the present work, emission spectroscopy and high-speed imaging were used to investigate the vapor plume behavior during a Yb:YAG laser pulse on the overlap joint between pure titanium and aluminum alloy A5754. A 15 ms long laser pulse was applied to the overlap joints, where titanium and then A5754 were placed on the top. Correlation of the obtained results with post-mortem observation of the impact zones and with a finite-element model of the keyhole evolution was performed. The combination of these approaches facilitated the development of a comprehensive phenomenological timelines of the processes, along with an evaluation of the efficacy of the employed online methods to discern the involvement of the bottom material with the melted zone. The considered configurations showed very different behavior: with reflective A5754 placed on top, the use of high laser power produced an intense keyhole propagation in bottom titanium plate, inducing rapid mix between the elements, while with titanium on top, the use of lower laser power produced prolongated keyhole stagnation at the interface with reflective A5754. High-speed imaging showed very fluctuating behavior of the plume, where the involvement of the bottom material was traduced either by a drastic drop of thermal and atomic emission after the keyhole tip enters the bottom A5754 plate, or by strong periodic bursts of Ti-rich jet after the keyhole reaches the bottom titanium plate. The results of emission spectroscopy were found in adequation with the involvement of bottom material into the melted zone, however, they are affected by plume fluctuations and by the pollution of the top plate by volatile elements.

The article describes properties of welds made of high wear resistance X120Mn12 steel obtained by the hybrid PTA-MAG (plasma transferred arc – metal active gas) method. The specimens were 8 mm thick rectangular (200 mm × 350 mm) sheets metal. The analyzed butt welds were made with the parameters selected according to the criterion of smallest cross-sectional area of welds and the narrowest HAZ (heat affected zone). The outcome of metallographic tests of weld, HAZ and parent material, hardness distribution and XRD (X-ray diffraction) patterns of selected areas are presented. The IIT (Instrumented Indentation Test) method was used to describe the distribution of mechanical properties shaped by thermal cycle annealing of the welding process. The investigation shows that the application of the PTA-MAG hybrid heat source for welding manganese steel enables the use of the filler material ER307 (AWS-A5.9). The hybrid PTA-MAG welding system has the relatively high potential to be an efficient alternative to welding standard processes for X120Mn12 steel due to the HAZ overheating limitation. The zone of high-risk weld cracking is the part of the HAZ close to the fusion area that has been reheated during weldment formation. Heat input about 0.6 kJ/mm is needed to provide full deep penetration butt weld without defects and with a vapor capillary of wide enough to cover the weld gap. The increase of hardness in the welded joint is smooth distributed and going up to 10% compared to the base material. The width of HAZ was <1 mm. Intensive carbides precipitation in HAZ has been avoided successfully.

The laser welding of Cu–Al alloys for battery applications in the automotive industry presents significant challenges due to the high reflectivity of copper. Inadequate bonding and low mechanical strength may occur when the laser radiation is directed toward the copper side in an overlap configuration welding. To tackle these challenges, a laser surface treatment technique is implemented to enhance the absorption characteristics and overcome the reflective nature of the copper material. However, elevating the surface roughness and heat-energy input over threshold values leads to heightened temperature and extreme weld. This phenomenon escalates the formation of detrimental intermetallic compounds (IMC), creating defects like cracks and porosity. Metallurgical analysis, which is time-consuming and expensive, is usually used in studies to detect these phases and defects. However, to comprehensively evaluate the weld quality and discern the impact of surface structure, adopting a more innovative approach that replaces conventional cross-sectional metallography is essential. This article proposes a model based on the image feature extraction of the welds to study the effect of the laser-based structure and the other laser parameters. It can detect defects and identify the weld quality by weld classification. However, due to the complexity of the photo features, the system requires image processing and a convolutional neural network (CNN). Results show that the predictive model based on trained data can detect different weld categories and recognize unstable welds. The project aims to use a monitoring model to guarantee optimized and high-quality weld series production. To achieve this, a deeper study of the parameters and the microstructure of the weld is utilized, and the CNN model analyzes the features of 1310 pieces of weld photos with different weld parameters.

The paper describes a high-current Gas Metal Arc Welding (GMAW) process using wire electrodes with diameters up to 4.0 mm for single-pass full penetration butt joint welding of 20 mm thick steel plates. Fundamental research aims to develop thick-wire GMAW into a high-efficiency method by identifying the limits of welding performance and achievable deposition rates. Current gaps in understanding include equipment requirements, process properties, application fields, and weld quality. The research project addresses these gaps through systematic investigations of basic technological analyses, application sample welding, and quality evaluations. The objective was to create a robust, cost-efficient gas-shielded high-performance welding technology with deposition rates comparable to Submerged Arc Welding. The fully mechanized, automatic welding setup included two parallel-connected welding power sources, one wire feeder and one high-power welding torch. Welding parameters and conditions were evaluated with the aim of achieving a high-quality weld. Optimal parameters were identified for one-sided single-pass welding on 20 mm thick plates. Validation of thick-wire GMAW for 20 mm thick high-strength steels was conducted via two-sided single-pass welding on S690Q grade plates. Testing of the weld joint included static tensile strength test (3x tensile specimen), a Charpy impact test at −40 °C (6x Charpy V-notch specimens respectively with notch position in weld metal, base material and heat-affected zone (HAZ)), microstructure examination and a hardness test. The lowest recorded impact energy was observed to be 50 J within the weld metal, in combination with hardness peaks in the HAZ reaching 415 HV1, and all tensile specimens failing outside the HAZ within the base material. The process achieved reliable, reproducible, and economical joint welding, meeting necessary mechanical-technological quality standards. The paper enhances the understanding of selected welding techniques tor thick plate joining and offers valuable industrial insights, demonstrating the technique's applicability and feasibility for high-strength applications.

The composition of filling-alloyed flux-cored wires is achieved by adjusting the filling components without changing the wire material. A disadvantage of this type is, that due to the high alloy concentrations in the filling, microscale segregation can occur in the melt pool. The ‘theory-of-two-melts’ is cited in the literature as the reason for this. This theory states that, due to the typical separate melting behaviour of filling-alloyed flux-cored wire, there are droplets of different compositions (melt A and melt B) which do not homogenise in the melt pool and cause these microscale segregations. To test this theory, the melting behaviour of filling-alloyed flux-cored wires and the homogeneity of the individual droplets were investigated in this work by using the indicator material P92 (9% chromium steel). For this purpose, the liquid melting droplets were separated and then analysed metallographically via optical microscope and chemically using energy dispersive X-ray spectroscopy. Based on this, the melting behaviour was then optimised by adjusting the wire temperature. For this purpose, two different methods (conductive heating before and after current contact nozzle – CCN) for wire preheating were analysed. On the one hand, the results show no deviations in the chemical composition between the individual drops. On the other hand, chromium-rich micro segregations could already be detected directly in the droplets. The ‘two-melt’ theory as the cause of the microscale inhomogeneities can therefore not be confirmed. However, it was shown that an increase in the wire temperature, regardless of the preheating system, leads to a reduction of the chromium-rich inhomogeneities in the droplets by up to 95%. This effect was strongest in the presence of a melting behaviour similar to solid wire, which could be realised by increasing the contact tip working distance to 70 mm.

Friction stir welding (FSW) is subjected to process-specific challenges including comparatively high process forces and tool wear resulting from thermomechanical stresses. As a result, the acting loads and the geometric-related tool wear can cause tool failure. The tool (shoulder) design, whether it is concave or flat, with or without geometrical elements, is mainly responsible for the related failure mechanism and tool life. Therefore, this study systematically analyzes the failure mechanisms as a function of the process temperature, during FSW of AA-6060 T66 using tools made of H13 tool steel, with different shoulder designs, namely a concave contour and a scroll contour. The mechanism responsible for tool failure was induced by repeated welding at rotational speeds of 4000 rpm and 2000 rpm, at process temperatures within the range of the secondary hardness maximum (552 °C and 555 °C) and below the temperature of the secondary hardness maximum (488 °C and 499 °C). The experimental investigation showed that reducing the rotational speed of the scrolled shoulder from 4000 rpm to 2000 rpm resulted in less wear and therefore an increase in tool life from 474 m to up to 1400 m. In this context, it has also been shown that the shoulder geometry affects the mechanism relevant to failure due to the free length of the probe.